The present invention relates to an imaging lens, a camera, and a portable information terminal device.
An imaging lens of the present invention is provided as a photographing optical system of a silver-halide camera, a digital camera, a digital video camera, and a portable information terminal device. A camera of the present invention is provided as a silver-halide camera, a digital camera, a digital video camera, and a portable information terminal device.
The use of digital cameras has become commonplace, and there have been various requests from users regarding digital camera functions. Among them, a high-quality compact camera with a relatively large image sensor having a diagonal length of approximately 20-45 mm and a high-performance single focus lens has gained great attention.
As to such a compact camera, in addition to having high performance and a large diameter, excellent portability, that is, miniaturization is in great demand.
In terms of high quality, in addition to having resolution corresponding to an image sensor of at least 12-24 megapixels, less comatic flare, high contrast, and no point image distortion in a peripheral part of an angle of view at an open aperture, less chromatic aberration and no occurrence of unnecessary coloring in a part having a large brightness difference, less distortion, and capability of drawing a straight line as a straight line, and so on are at least needed.
In terms of the large diameter, due to the necessity to make different from a general compact camera with a zoom lens, an F-number of less than F3 at least is needed.
In terms of miniaturization, in a high-resolution compact camera, since a large image sensor is used relative to the size of the camera body, an actual focus length is longer than that of a compact camera having a small image sensor. Therefore, in order to achieve miniaturization of the design while having the high-resolution image sensor, it is necessary to shorten the entire length of the imaging lens in the camera.
Additionally, in terms of an angle of view of a photographing lens, many users demand a rather wide angle, and it is preferable that a half angle of view of an imaging lens be equal to or more than 35 degrees. The half angle of view of 35 degrees is equivalent to a focal length of about 31 mm in a 35-mm silver-halide camera using traditional 35-mm silver-halide film (so-called Leica format silver-halide film).
An image sensor of a digital camera has a color filter and a microlens per pixel, and therefore, it is preferable that a position of an exit pupil be distant from an image plane, and a peripheral light beam be approximately vertically incident onto an imaging plane.
In order to achieve this object, retrofocus-type imaging lenses are suitable, and conventionally many of those have been employed. However, when a retrofocus lens is employed, an entire length of an imaging lens (a length from a surface on a most object side to an image plane) tends to be longer.
Recently, in a relatively large image sensor having a diagonal length of 20-45 mm, due to improvement and optimization of an on-chip microlens, development of image processing, and the like, a slightly inclined incident state has been allowable as a state where a peripheral light beam is incident onto an imaging plane.
That is, in particular, a system capable of sufficiently accepting an angle between a principal ray and an optical axis at a maximum image height of about 35-45 degrees is constructible, and it is possible to choose types of lenses more suitable for miniaturization without restricting a condition of vertical incidence of a peripheral light beam.
From the above viewpoint, as types of lenses more suitable for miniaturization than the retrofocus-type, there are an approximately-symmetric-type, and a telephoto-type in which a lens group having a negative refractive power is arranged on an image side. Conventional examples of imaging lenses of this kind are disclosed in the following patent documents.
Japanese Patent Application Publication Number H08-313802 discloses an imaging lens of an approximately-symmetric-type wide lens having a sufficiently large diameter. However, an entire length of the lens, or an entire thickness of the lens (length from a surface on a most object side to a surface on a most image side of a lens system) is large, and therefore, there is room for improvement in terms of miniaturization.
Japanese Patent Application Publication Number H11-326756 discloses an imaging lens having a constitution close to an approximately-symmetric-type, and achieves a wide angle that exceeds a half angle of view of 50 degrees; however, its smallest F-number is F4-4.5, which is large.
Japanese Patent Application Publication Number 2005-352060 discloses an imaging lens having a small entire length/thickness of the lens; however, since a back focal distance is short, a diameter of a lens close to an image plane is large. Therefore, there is room for improvement in terms of miniaturization.
Japanese Patent Application Publication Number 2012-008347 discloses an imaging lens that excels in an angle of view, an F-number, and imaging performance; however, a feature of a retrofocus lens strongly remains, and therefore, there is room for improvement in terms of miniaturization.